Air gap shield for loud-speakers



Sept. 23,v 1941. J. P. QUAM .l y AIR GAP SHIELD FOR LOUD-SPEAKERS 4Filed June 17, 1959 zz/@i2 Q/Qies Z9 @aa/ y M M particles into the airgap,

Patented Sept. 23, 1941 AIR GAP SHIELD FOR LOUD-SPEAKERS James P. Quam,Chicago,

Nichols Company, Chicago,

Missouri Ill., assignor to Quam- Ill., a corporation of Application June17, 1939, Serial No. 279,694.

1 Claim.

My invention relates in general to loud speakers and more in particularto a shield for impeding the movement of foreign magnetic materials intothe air gap of a permanent magnet dynamic loud speaker. The shield hasbeen satisfactorily applied in a commercial embodiment of the inventionto the loud speaker of my izo-pending application, Serial No. 215,484.

In a loud speaker employing a movable voice coil, it is desirable toprovide an air gap which is just wide enough to permit the movement of avoice coil 'therein Without rubbing on the pole piece structure definingthe inside and outside edges of the air gap. The narrow air gap isdesirable because the more narrow the air gap the greater the magneticeld at the air gap to move the voice coil therein. With a narrow air gapforeign particles which move into the air gap from outside the magnetstructure may impede the movement of the voice coil so as to affect theoperation of the speaker. Although it is desirable to prevent theintroduction of all foreign those which cause the greatest trouble inpermanent magnet speakers are particles of magnetic material which moveinto the air gap and are retained therein by the permanent magnetpole-piece structure. The introduction of such undesirable foreignmaterial into the air gap in the past has generally been prevented bythe use of a yieldable member supplied in cloth, felt, or paper, whichwas secured to the moving portion of the speaker such as the spider forthe voice coil, or the diaphragm and to the outer pole piece tosubstantially cover the air gap. This structure, however, Was expensiveand added to the mass of the moving portionV of the speaker, so that theinertia of such moving portion was increased to cause a loss inoperating efficiency. Furthermore, not only was the original cost forsuch foreign-material-proofing structure high in respect to material andlabor, but this structure increased the cost of repairing aloud speakerin the event that it was necessary to remove the diaphragm, voice coil,and spider.

An object of the present invention is to provide an improved loudspeaker.

A further object of my invention is to provide an improved dynamic loudspeaker.

Another object of my invention is to provide an effective, sturdy, andsimplified shield to impede the movement of foreign particles into theair gap of a loud speaker.

It is also an object of my invention, to provide a magnetic shield forthe air gap of a dynamic speaker to particularly prevent theintroduction therein of magnetic particles.

A still further object of my invention is to provide a shield which willimpede the movement of foreign particles into the air gap of a dynamicspeaker from the outside thereof, but with such shield, being entirelyindependent of the moving portion of the speaker and not in any Wayadding to the mass thereof.

A feature of my invention is the provision of magnetic means forpreventing the introduction o-f foreign magnetic particles into the airgap of a loud speaker which does not in any Way require useful magneticflux for its energization.

Other objects and features of my invention will b-e apparent from thefollowing description taken with the drawing in which:

Fig. 1 is a side elevation of a complete loud speaker embodying myinvention.

Fig. 2 is an enlarged fragmentary front elevation view takenapproximately on the line 2 2 of Fig. l.

Fig. 3 is a vertical sectional View of the magnet portion of the loudspeaker having the magnetic shield secured thereto.

Fig. 4 is a detail View in perspective of the magnetic shield.

In practicing my invention, I provide a member of magnetiaable materialfor securing to the magnet portion of a loud speaker adjacent the airgap for such speaker, and being provided in such shape and of such asize that it normally surrounds the air gap in a manner to impede themovement of undesired foreign material into the air gap. Such member ofmagnetizable material, in one commercial embodiment of the invention isprovided in a stamped-out one-piece member having raised portions'thereon to act as physical barriers to the movement of foreignparticles and to generally increase the effective exposed area of theshield. Althoughv the member acts primarily to impede the movement ofundesired magnetic materials into `the air gap, it may be convenientlydesignated as a dust shield for the air .gap of a loud speaker. When thedust shield is secured to the outside of the speaker magnet structuraitis magnetized by the contact and acts in turn to attract to and retainthereon foreign magnetic material so that it will not get into the airgap to interfere with the operation of the voice coil therein.

Referring now to the drawing, a complete permanent magnet dynamic loudspeaker is illustrated'in Fig. 1 and includes a permanent magnetstructure ycomprising a ring IU, with plates li and I2 secured thereto.These three members are all of permanent magnet material, and whensecured together, form the so-called pot for the speaker. Plate I I, asshown in Fig. 3, is provided with a central opening I3 and extendingtherein is a core or inner pole piece I4, which in turn is rigidlysecured in a corresponding aperture in the magnet plate I2. The movablediaphragm I6 of the loud speaker is secured in the usual manner to adiaphragm frame I'I or basket which is welded to the face of the magnetplate Il in a position centered with'reference to the aperture I3.

diaphragm I6 `and includes a paper form I8 (Fig.

2) with a wire coil IS wound thereon. The voice coil moves in an annularair gap A defined on the outside by the edge of the aperture I3 in themagnet plate II and on the inside by the cylindrical surface of the coreI4 as is shown clearly in Figs. 2 and 3. f

A magnetic eld is permanently maintained at the air gap and the strengthof such field is inversely proportional to the square of the Width ofthe air gap, so that it is desirable to have this air'lgap as narrow aspossible. As a result, the air gap is normally of a' width such that thevoice coil will conveniently move in a direction parallel to the axis ofthe core M'just out of engagement with both sides of the gap. A spider2| of yieldable insulating material is secured to the diaphragm or voicecoil substantially at the juncture of the two to maintain the voice coilcentered Ain the air gap.

Because of the danger of foreign material getting into a'narrow air gapand interfering with the movement of the voicey coil therein, air-gapsin dynamic loud speakers have in the past often been wider than theyshouldrbe for the most eincient operationof the speaker. As a result ofmy invention as ldisclosed in the copending ap- A voice coil is securedto the apex of theV ing power by virtue of the use of the shield. Itisto be understood also that in practicing my invention I may provide theshield in material which is permanently magnetized independently of thepermanent magnetization it acquires by being mounted on the permanentmagnet of the speaker. The rims 24 and 26 increase the effectivemagnetized area of the shield and provide not only la physical barrierto the movement of undesiredmagnetic material into the air gap, but alsoa magnetic barrier. The rims operating as a double physical barrier, soto speak, to the movement of magnetic particles across the plate II, areimportant inasmuch as continuous jarring of a speaker in transit or thelike may cause movement even over a magnetized surface, but it issubstantially impossible to jar particles past the rims or raisedportions.

In manufacturing the complete speaker the magnet pole structure isnormally welded together, with the basket I 'I being welded to the plateII either before or after welding the plate II to the ring I0. Eitherbefore orafter welding the basket to the plate II, the shield 22 iscentered with reference to the aperture I3, or the air gap A and iswelded to the plate I I. When the entire metal frame structure iscomplete it is immersed in a paint bath and processed'as Vdescribed inmy copending lapplication mentioned above. This substantially eliminatesthe possii bility of foreign magnetic material from getting plicationpreviously noted, I have substantially eliminated the possibility ofundesirable foreign material getting into the air gap from' the insideof the pot, and as stated above, fabric dust caps have been secured overthe air-gaps in such speakers to prevent entrance from the outside.However, as mentioned, such dust caps add to the mass of the movingportion of the speaker and are expensive.

Ordinary dust havingno magnetic material in it is normally blown out ofthe air gap simply by the propulsion of the diaphragm and voice coil inthe operation of the same so that this is not serious. However, foreignparticles of magnetic material which move or fall into the air gap re-vmain therein on the permanently magnetized pole structure and mayseriously impede the movement of the voice coil. Fig. 4 illustrates indetail a shield of Vmagnetizable material for attracting to andretaining thereon foreign particles of magnetic material so thatsuchparticles will not get into the air gap A from the outside. Theshield is secured to the magnet plate I I surround` ing the air gap Aand becomes magnetized by virtue of its contact therewith. Theillustrated shield 22 comprises a piece of magnetizable sheet metalwhich is blanked and formed to'provide an annular opening 23 forsurrounding the air gap A, .with a substantially right-angled rim 24vdefining the edge of the opening 23, Iand a similar rim 26 at thecircumference of the shield. The shield is magnetized by leakage uXfromA the air gap. Utilizing leakage flux for magnetizing the shieldcauses no additional loss in the magnetic flux at 'the air gap and henceno loss in operatinto the air gap from the inside. After this thediaphragm, voice coil, and spider are assembled onto the frame. Theshield 22 Vbecomes magnetized whenever the magnet structure of thespeaker is magnetized and is immediately effective.

The shield 22 has no physical connection with the moving portion of thespeaker so that it does not in any way add to the mass of the movingportion and hence reduce the operating eniciency thereof. Furthermore,the shield itself is sturdy and simple and represents a very small costso far as material vand labor is concerned. The shield 22 is not subjecttoatmospheric conditions such as the fabric dust-proof shield of theprior art. Because of its position on the speaker frame structure out ofcontact with the moving portion, and because of its imperviousness toinjury, the use of the shield 22 reduces the cost of any repair of thespeaker which would necessitate the removal of the diaphragm, voice`coil land spider.

Although my invention has been illustrated in its'preferred embodiment,it is understood that it is not limited thereby, but the invention islimited only by the scopeof the appended claim.

I claim:

In a loud speaker including a pole piece having an annular aperturetherein with the annular wall of said aperture forming one wall of theannular air gap in said speaker, and a member movable in said annularair gap,rmeans for arresting the movement of foreign material linto saidannular air gap including a magnetizable member mounted on said polepiece adjacent the annular air gap, said magnetizable member comprisingan annular body portion having an annular ring on the insidecircumference and an annular ring on the outside circumference thereof,said annular rings acting magnetically and physically to arrest themovement of foreign particles over the 'pole piece toward the air gap.

JAMESP- QUAM-

